1. Field of the Invention
The present invention relates to an improvement in anti-icing systems for aircraft jet engine propulsion systems and more particularly, but not by way of limitation, to an improved method and arrangement for a swirling anti-icing system for an inlet of an aircraft jet engine which provides for improved mixing of the injected hot gas and enhanced pumping of the air within such inlet.
2. Description of the Prior Art
Safety is a primary concern in the design of power propulsion systems for aircraft applications. The formation of ice on aircraft wings, propellers, air inlets of engines, etc. has been a problem since the earliest days of heavier-than-air flight. Any accumulated ice adds considerable weight, and changes the airfoil or inlet configuration making the aircraft much more difficult to fly and in some cases has caused loss of aircraft. In the case of jet aircraft, large pieces of ice breaking loose from the leading edge of an engine inlet housing can severely damage rotating turbine blades or other internal engine components and cause engine failure.
Many attempts have been made to overcome the problems and dangers of aircraft icing. The inlet area of nacelles for aircraft propulsion systems have been the focus of a significant amount of research and development within the aircraft industry. For example, proposals have been made, as described in U.S. Pat. No. 2,135,119 to mechanically vibrate external surfaces to break ice loose or, as described in U.S. Pat. No. 3,549,964, to generate electro-magnetic pulses in the aircraft skin to break ice loose. These systems, however tend to be heavy and complex and to only remove existing ice, rather than prevent ice formation.
Heating areas of the aircraft prone to icing has been suggested many times. The heating schemes suggested range from microwave heating as suggested by U.S. Pat. No. 4,060,212 to feeding hot gases through holes in the skin, as suggested by U.S. Pat. No. 4,406,431, to resistance heating of the surfaces, as in U.S. Pat. No. 1,819,497, to actually burning fuel adjacent to ice-prone surfaces, as described in U.S. Pat. No. 2,680,345. While each of these methods have some advantages none had been truly effective.
One of the most common anti-icing techniques has been the ducting of hot gases into a housing adjacent to the likely icing area. Typical of the patents describing such hot gas techniques are U.S. Pat. Nos. 3,057,154; 3,925,979; 3,933,327 and 4,250,250. In each case, the hot gas conduits simply dump hot gases into a housing, such as the leading edge of a jet engine housing or a wing leading edge. While often useful, these systems are not fully effective due to the complexity of the hot gas duct system.
A typical design for a transport aircraft engine nose cowl ice protection system was the double skin, spray bar configuration which employed an annular duct installed within the nose cowl xe2x80x9cD-ductxe2x80x9d space. Hot air jets issuing from small orifices, or piccolo holes, on the spray tube were directed onto the entrance into the D-duct into double skin passages along the upper and lower lip surfaces, away from the high-light portion of the nose lip. Hot air was then forced to flow through narrow gaps between the outside skin and the inner skin, transferring heat to the outer skin. While some heat effectiveness was achieved by the double skin design it was at the expense of high manufacturing cost and weight penalty, associated with the required chem-milling process to produce the inner skin and to provide the spray tube arrangement.
A substantial advance in anti-icing systems was made in U.S. Pat. No. 4,688,745 entitled xe2x80x9cSwirl Anti-Ice Systemxe2x80x9d and issued to Rosenthal. This patent provided for the circulation of heated gases within the circular leading edge of a jet engine housing in a rotational swirling motion to prevent ice build up thereon. Hot gas, such as air from a hot, high pressure section of the jet engine was directed to the D-duct inlet interior through a conduit that enters the annular leading edge housing through a bulkhead closure. The conduit is then turned about 90xc2x0 to a direction tangential to the center line of the leading edge annulus. The hot gases exiting an injection nozzle provided as an outlet of such conduit entrain the cooler air in the circular leading edge and cause the much larger mass of air to swirl circularly around the interior of the D-duct of the annular housing. The entering hot gas heats the mass of air to an intermediate but still relatively hot temperature which then uniformly transfers heat to the skin of the leading edge without leaving any relatively cold areas and preventing the formation of ice on the inlet lip. A fraction of entrained heated air equal to the flow rate of injected hot gas is exhausted from such housing.
While this anti-icing system represented a significant advancement over the prior art and has been widely accepted in the aircraft industry, there are areas of the system that could be improved. It has been found that as the jet nozzle injects a flow of under-expanded hot air into the annular nose cowl or nose lip of the inlet that the complete mixing of the two masses of air is somewhat delayed during the rotational swirling action and results in a xe2x80x9chot spotxe2x80x9d or area of elevated temperature on the outer lip skin of the nose lip at a position downstream of such injection. This area of elevated temperature in the noselip then presents a constraint in the design of an anti-icing system according to teachings of such patent since such design must take into account such actual conditions such as a day having an elevated ambient temperature, a low altitude location for the aircraft that is to take-off, a high engine power setting, and a failure to an open position of an anti-ice valve provided in the conduit supplying hot gas from the engine as required.
Thus, there is a continuing need to improve aircraft engine housing icing prevention and to particularly improve the anti-icing system of U.S. Pat. No. 4,688,745 by enhancing the mixing of the injected hot gas and the mass of swirling air contained within the D-duct noselip of an aircraft jet engine housing and to improve the performance of the anti-icing system and to lessen design constraints imposed by the area of elevated temperature in the nose lip downstream of the point of injection of the hot gas.
It is a general object of this invention to provide an improved swirling rotational anti-icing system for the nose cowl of a transport aircraft jet engine which enhances the mixing of injected hot, high pressure gas with the larger mass of air within the nose cowl.
It is a further general object of this invention to improve the mixing of hot, pressure gas with the mass of air within the nose cowl and thereby materially reduce any tendency for the injection of such hot, high pressure gas to create an area of elevated temperature in the nose lip at a position downstream of the injection of such hot gas under severe design conditions.
A further general object of the invention is to enhance the pumping action that the injected hot gas has on the ambient air within the nose cowl.
A still further general object is to enhance the circumferential uniformity of noselip temperature and noselip heat rejection to improve the anti-icing efficiency and to prevent ice accumulation on the exterior of the noselip upstream of the point of hot gas injection.
Another general object of the invention is to enable reduction in the noselip cross sectional area and to thereby increase the effectiveness of engine noise attenuation by the inlet by permitting a greater area of sound attenuating structure to be used in the inner barrel of the air inlet.
The foregoing has outlined some of the more pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and application of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or by modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.
Briefly stated, the present invention contemplates overcoming the above noted problems of the prior art and others by the anti-icing system of the present invention which also further improves upon the performance of the generally accepted previous highest standard for an anti-icing system. The present invention contemplates at least one conduit means which directs hot gas from a source of hot, high pressure gases, such as a hot high pressure region of a jet engine to an annular single skin housing or nose lip of the air inlet of an aircraft engine, the air inlet having an acoustically treated inner barrel. The hot gas conduit enters the nose lip through a transversely arranged bulkhead and then is immediately bent approximately 90xc2x0 and further shaped as a nozzle so as to inject a high speed hot gas jet into the mass of air within such housing at a high velocity substantially along a tangent to the nose lip interior. The arrangement of the nose lip and bulkhead is sometimes referred to in the art as a xe2x80x9cD-duct.xe2x80x9d The outlet nozzle of the conduit is provided with a plurality of circumferentially arranged triangularly shaped tabs that extend aft from the nozzle and are canted inwardly into the exiting flow of hot air to create large scale longitudinal vorticity or transverse stirring aspect to the exiting flow of hot air.
This turbulent flow of injected hot air then mixes more quickly than in the past with the larger mass of lower velocity air in the housing and begins to entrain the larger air mass and brings the velocity of hot gas and air to an intermediate velocity. Likewise, heat is transferred from the injected flow of hot air to the larger mass of air to bring the resultant intermixed flow of rotating swirling flow of air up to an intermediate, but still high, temperature. In effect, energy is conserved by trading high velocity and high temperature in the small mass of injected hot gas for lower velocity and a slightly lower temperature in a larger mass of air/ hot gas mixture which still has a temperature sufficiently high to relatively uniformly heat the skin of the noselip to a temperature which precludes the formation of ice on the external surface of the nose lip.
Thus, the improved mixing of the hot gas and the mass of internal air also precludes the formation of an area of unduly elevated temperature on the nose lip at a position downstream of the injection of hot gas and precludes formation of an area of unduly cold temperature on the noselip at a position upstream of the injection of hot gas and improves uniformity of heat rejection around the D-duct circumference. An outlet means is provided in the nose lip bulkhead construction to permit the exhaust of a flow of mixed air from the D-duct that is equal to the flow of hot air injected into such housing. An alternative outlet means and presently preferred structure is to provide an outlet means through slots provided in the noselip itself, preferably at the bottom outer portion of such noselip. This construction has been found to be a very efficient and simple anti-icing system that improves the effective heat transfer that takes place from the large mass of heated moving air to the D-duct wall while retaining mechanical simplicity. There are no moving mechanical parts or electrical parts to jam or burn out.
The foregoing has outlined rather broadly the more pertinent and important features of the present invention in order that the detailed description of the invention that follows may be better understood so that the present contribution to the art may be more fully appreciated. Additional features of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the disclosed specific embodiment may be readily utilized as a basis for modifying or designing other structures and methods for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions and methods do not depart from the spirit and scope of the invention as set forth in the appended claims.